Generally, a polyimide (PI) film is a film of polyimide resin. Polyimide resin is a highly heat-resistant resin obtained by subjecting aromatic dianhydride and aromatic diamine or aromatic diisocyanate to solution polymerization to prepare a polyamic acid derivative, which is then subjected to ring closure and dehydration at high temperatures to imidize it.
Polyimide resin, which is insoluble, infusible and resistant to very high heat, has superior properties regarding such as thermal oxidation resistance, heat resistance, radiation resistance, low-temperature resistance, and chemical resistance, and is thus used in various fields of application, including advanced heat resistant materials such as automobile materials, aircraft materials, or spacecraft materials, and electronic materials such as insulation coating agents, insulating films, semiconductors, or electrode protective films of TFT-LCDs. Recently, polyimide resin is also used for display materials, such as optical fibers or liquid crystal alignment layers, and transparent electrode films, in which conductive filler is contained in the film or is applied onto the surface of the film.
However, polyimide resin is disadvantageous because it has a high aromatic ring density, and thus is colored brown or yellow, undesirably resulting in low transmittance in the visible light range. Polyimide resin suffers because light transmittance is decreased attributable to the yellow-like color thereof, thus making it difficult to apply the polyimide resin to fields requiring transparency.
In order to solve such problems, attempts to realize methods of purifying monomers and a solvent to high purities in order to achieve polymerization have been made, but the improvement in transmittance was not large.
U.S. Pat. No. 5,053,480 discloses a method of preparing a polyimide resin using an alicyclic dianhydride component instead of the aromatic dianhydride. According to this method, when the polyimide resin is in a solution phase or a film phase, transparency and color are improved, compared to the results of the purifying methods. However, limitations are imposed on improving transmittance, and thereby high transmittance is not realized, and further, the thermal and mechanical properties are deteriorated.
In U.S. Pat. Nos. 4,595,548, 4,603,061, 4,645,824, 4,895,972, 5,218,083, 5,093,453, 5,218,077, 5,367,046, 5,338,826, 5,986,036, and 6,232,428, and Korean Unexamined Patent Publication No. 2003-0009437, there have been reports related to the preparation of a novel polyimide, which is improved in terms of transmittance and color transparency within a range inside which thermal properties are not greatly decreased, using aromatic dianhydride and aromatic diamine monomers, having a linker, such as —O—, —SO2—, or CH2—, a bent structure due to connection not at the p-position but at the m-position, or a substituent, such as —CF3. However, the polyimide thus prepared is evaluated to have mechanical properties, yellowing index, and visible light transmittance which are inadequate for use in semiconductor insulating films, TFT-LCD insulating films, electrode protective films, and flexible display substrates.
Moreover, in the case where the polyimide resin is colorless and transparent but has a high coefficient of linear thermal expansion (CTE), the degree of expansion and contraction of the polyimide film depending on the changes in temperature in the TFT process is increased, undesirably causing damage to an inorganic film used in devices and deteriorating the capability of devices. Therefore, with the goal of using the polyimide resin as substrates for TFTs, substrates for color filters, and alignment layers, the polyimide resin must have a low CTE while being colorless and transparent.